People and their egos.

Safety. Or rather the lack of it. Both sides should feel safe from receiving any kind of harm from the other, whether that's from lying, cheating, not having each other's backs, running away from each other... So many things. They have to be 100% safe with each other, and know that they are as well. They should know that no matter what happens one won't harm the other, and they'll always be there for each other no matter what. So yeah, safety.

Taking things for granted.

Its relational traumas, fear of pain, fear of vulnerability, lack of differentiation between emotions and behaviour/self awareness of behaviours, circumstance, so many factors can play into something as complicated as that.

Assumptions, unrealistic expectations, being raised on Disney princess cartoons, lack of communication, inability to apologize or be empathic, any relationship based solely on great sex and excess drinking/drugs.

Two minds needing to synchronize and communicate as one. There has to be the base of overlap in understanding and also enough difference to avoid stagnation.

Unreasonable expectations...

Self-centerdness...

Continueous lying...

Selfishness, control, manipulation, ego

I’m sure it’s been said, but trust.

That's by design. Capitalism benefits from atomization of society and capitalism is threatened by resolution of conflicts.

Misunderstandings

tensile strength

Personally, lack of effort.

Insecurities and expectations

People can be selfish and forget we have to give as well as receive

Ego and selfishness

(Glass) ships.

Fragile egos